This clinical study distinguishes itself by being the first to evaluate the vascular changes induced by esterified estrogens 0.625 mg/day alone and with the addition of methyltestosterone or medroxyprogesterone acetate, as opposed to conventional-dose conjugated estrogens 0.625 mg/day or equivalent. This study will specifically evaluate changes in nitric oxide physiology induced by estrogen replacement therapy (ERT) and combination therapy (esterogen-progesterone and estrogen-androgen) at conventional doses, as opposed to acute, high-dose regimens, which are likely to be less representative of the clinical setting.